This invention relates generally to a coordinate position digitizing system using pick-up means, such as a magnetic flux detecting means, which is movably set on a position determining plate to produce a signal indicative of position on the plate. More particularly, it is directed to a system for obtaining positional information defining a position of the pick-up means on a position determining plate on which a plurality of parallel conductors selectively energized are disposed, based on the strength of a signal derived from the pick-up means when the pick-up means is brought close to the plate.
There has been previously proposed a position determining apparatus utilizing magnetic flux changes as shown in FIG. 1. Referring to FIG. 1, a terminal 1 is connected to a power supply (+B) producing a certain DC voltage, while a terminal 2 is grounded. One end of each of a plurality of (five in the example shown) parallel conductors 5.sub.1 -5.sub.5 shaped into rectangular form is connected to a common connecting line 4, which is connected to the terminal 1 via a current-limiting resistor 3. The common connecting line 4 and the conductors 5.sub.1 -5.sub.5 are disposed on a position determining plate P. The other ends of the conductors 5.sub.1 -5.sub.5 are respectively connected to first ends of switches 6.sub.1 -6.sub.5 in a switch driving circuit 6, and second ends of the switches 6.sub.1 -6.sub.5 are connected together to the terminal 2. The switches 6.sub.1 -6.sub.5 in the circuit 6 are actuated in such a manner that only one of them is closed at a time by clock pulses a, as shown in FIG. 2A, which are supplied to the circuit 6 from an oscillator 7, so that a DC voltage obtained across the terminals 1 and 2 is supplied to the rectangular conductors 5.sub.1 -5.sub.5 in turn. A frequency divider 8 acts to deliver output pulses, each of which is used as a start pulse b as shown in FIG. 2B, in the ratio of one for every five input clock pulses from the oscillator 7. The output end of the frequency divider 8 is connected to the start terminal 9A of a counter 9, while the output end of the oscillator 7 is connected to a clock input terminal 9B of the counter 9. An amplifier circuit 11A having high input impedance is provided for amplifying a pulse signal which is produced by a pick-up coil 10 when the pick-up coil 10 is brought close to the plate P having the conductors 5.sub.1 -5.sub.5, and supplying an output signal c, such as shown in FIG. 2C, to a polarity detecting circuit 11B. The polarity detecting circuit 11B produces an output pulse used as a step pulse d, as shown in FIG. 2D, for every two successive pulses of the same polarity in the output signal c from the amplifier circuit 11A. The output end of the polarity detecting circuit 11B is connected to the stop terminal 9C of the counter 9. The output terminal 9D of the counter 9 is connected to a memory 12 which is supplied with output data obtained from the counter 9 and delivers the data to a signal processing circuit (not shown in the drawing Figs.).
In the operation of the prior art apparatus constructed as described above, it is assumed that the pick-up coil 10 is located at a point x on the plate P, that is, within the loop formed by the conductor 5.sub.3, and the switches 6.sub.1 -6.sub.5 in the switch driving circuit 6 are successively closed corresponding to each of the leading edges of the clock pulses a produced by the oscillator 7. When the switch 6.sub.1 is closed, the frequency divider 8 supplies the first start pulse b to the start terminal 9A of the counter 9, so that the counter 9 starts operating. Then, the conductors 5.sub.1 -5.sub.5 produce a variable magnetic field, and the pick-up coil 10 located at the point x on the plate P detects variations in the magnetic flux of the magnetic field and produces the pulse signal in response to the variations in the magnetic flux. This pulse signal is amplified by the amplifier circuit 11A to produce the output signal c from the latter. When the conductor 5.sub.3 is supplied with the DC voltage to produce a direct current flowing therethrough, that is, the switch 6.sub.3 is closed, the output signal c takes particular pulses different in phase from the pulses taken when the switch 6.sub.1 or 6.sub.2 is closed. The polarity detecting circuit 11B is operative to discriminate these particular pulses by detecting two successive pulses having the same polarity in the output signal c, and whenever such two successive pulses of the same polarity are detected, the polarity detecting circuit 11B produces the stop pulse d and supplies it to the stop terminal 9C of the counter 9. Thus, the counting operation of the counter 9 is initiated upon the leading edge of each start pulse b and stopped upon the leading edge of each stop pulse d. The result of the counting obtained by the counter 9 during the period from the leading edge of the start pulse b to the leading edge of the stop pulse d is transferred to the memory 12. In this specific example, the counter 9 starts counting at the leading edge of the first one of the clock pulses a and stops counting at the leading edge of the third one of the clock pulses a, and the result of counting during a period of time t equivalent to the duration of two cycles of the clock pulses a is supplied to the memory 12 in the form of data corresponding to the positional information concerning the point x.
When the pick-up coil 10 is moved from the point x to any other point on the plate P, the output signal c appearing at the output end of the amplifier circuit 11A has a waveform which is the same as a waveform obtained by shifting the waveform shown in FIG. 2C to the right or left along the abscissa representing time. This causes the time point at which two successive pulses of the same polarity in the pulse signal c are detected to be also shifted, so that the instant at which the stop pulse d is produced by the polarity detecting circuit 11B is made earlier or later. In this manner, new positional information concerning the position of the pick-up coil 10 on the plate P is obtained.
The aforementioned conventional apparatus determines the position of the plate P where the pick-up coil 10 is brought near based on the strength of the polarity of the pulses in the output signal c appearing at the output end of the amplifier circuit 11A in response to the pulse signal induced in the pick-up coil 10, and therefore the conventional apparatus can simply derive the positional information indicating one of the loops formed by the conductors 5.sub.1 -5.sub.5 in which the pick-up coil 10 is located. Accordingly, more specific positional information concerning the position of the pick-up coil 10, that is, positional information representing the detailed position of the pick-up coil 10 within one of the loops, cannot be obtained by the conventional apparatus. The conventional apparatus encounters the disadvantage that the resolution in detecting a position thereby is unsatisfactorily low. Further, in case the pick-up coil 10 is located between two parallel and neighboring conductors through which the DC current flows in apposite directions and which are the portions of two of the conductors 5.sub.1 -5.sub.5, there arises a possibility of malfunction. Consequently, the space between such two parallel and neighboring conductors is required to be made as narrow as possible, but this ideal is not attained because of the limitations in manufacturing. This results in that some locations unavoidably remaining at which the position of the pick-up coil 10 is detected in uncertain fashion.
When it is desired to obtain positional information defining the position of the pick-up coil 10 in two different directions, for example, the horizontal and vertical directions, a coordinate position digitizing system comprising the position determining apparatus shown in FIG. 1 and an additional position determining plate P' accompanied with another switch driving circuit which are similar to the position determining plate P and the switch driving circuit 6 shown in FIG. 1, respectively, is provided. In such a coordinate position digitizing system, two position determining plates P and P' are so disposed that the conductors on the position determining plate P are perpendicular to the conductors on the position determining plate P' and made operative alternately in order to produce, for example, horizontal positional information and vertical positional information, respectively.
However, in such a prior art coordinate position digitizing system using two position determining plates on which the conductors are mounted, since two independent switch driving circuit arrangements are provided for supplying the DC current to the conductors on the two position determining plates P and P' in turn, there is the defect that the configuration of the switch driving circuit arrangements is made very complicated.